The ENDO Study is a collaborative epidemiologic study whose goal is to assess the relation between exposure to environmental chemicals and lifestyle factors that may affect womens reproductive health. Particular focus will be on the relation between hormonally active chemicals and risk of developing endometriosis considering womens lifestyles. Endometriosis is a condition in which endometrial tissue is found outside the uterus and can be associated with pelvic pain, menstrual changes and infertility. Approximately 10% of women aged 18-44 years may be affected, and many cases may be currently undetected. To date, the cause of endometriosis is unknown and, increasingly, an environmental etiology is suspected for some affected women. To answer questions about the effect of the environment and lifestyle on womens gynecologic health, the ENDO Study will use an operative cohort design. Women aged 18-44 years residing in geographically defined areas of California and Utah who are undergoing laparoscopy or laparotomy are eligible for inclusion in this study, along with women who reside in geographic areas served by participating hospitals and clinics. Participation in the study asks women to: 1) complete an interview to provide information about lifestyle and medical history; 2) complete an anthropometric assessment to measure fatness and 3) provide blood and urine samples for the measurement of chemicals. Women undergoing surgery will be asked permission for the surgeon to remove fat, peritoneal fluid and endometrial tissue for laboratory analysis to help identify the cause(s) of endometriosis. Several environmental chemicals are associated with endometriosis in both clinical and population cohorts.